1. Field of the Invention
The invention relates to an image forming apparatus for analyzing input image information and forming an image and, more particularly, to an image forming apparatus having fixing means for thermally fixing a toner image which has been transferred to a recording medium.
2. Related Background Art
Hitherto, in such kind of image forming apparatus, after a toner image formed on a photo sensitive material was transferred to a recording medium, an image is fixed on the recording medium by fixing means such as a fixing device comprising a thermal roller and a pressing roller and the recording medium is subsequently discharged.
In the image forming apparatus having such a fixing device, in order to avoid a fixing jam, an image margin of a predetermined amount, for example, about 5 mm from a front edge of the recording medium is forcedly set and the image is formed. Unless such an image blank is provided, in the case where, for instance, a toner image corresponding to an image which has fully been painted in black was transferred from the front edge of the recording medium, the recording medium after completion of the fixing of the image is wound around a fixing roller and some separating means such as a separating claw or the like is needed, causing the fixing device to increase in size. Therefore, it is the present situation that the technical problem is solved by forcedly setting the image blank as mentioned above.
FIGS. 6 to 8 are diagrams showing image patterns which are generated from such a kind of image forming apparatus. It is assumed that the upper portion in each of the diagrams coincides with the paper feeding direction.
FIG. 6 corresponds to an image output pattern having a blank (region in which no toner exists) of about 30 mm in each of the upper and lower portions of the paper and a blank of about 25 mm in each of the right and left portions. The image output pattern occupies most of a general document. Almost of a document, book, and other printed matter formed by a document forming apparatus also correspond to such a pattern.
FIG. 7 corresponds to an image output pattern in which information such as a character pattern fully exists up to a portion near the outer periphery of the paper. For example, FIG. 7 relates to the case where the telephone number of the transmitting side, the date of transmission, the number of pages transmitted, and the like are printed in the leading portion as in a facsimile image. Or, FIG. 7 relates to the case where a large amount of information is printed up to the edge of the paper as in a special application.
FIG. 8 corresponds to an image output pattern in which a black pattern exists in the front edge portion in the feeding direction of the paper. For example, FIG. 8 relates to the case where information near black exists in a leading portion of an image such as copy of a graphic pattern or color image including a photograph, a shadow of a peripheral portion occurring in the copying operation by a copying apparatus, or the like.
In case of the image information the content of which provides an image output pattern like shown in FIG. 6, no problem occurs, even when a blank of about 3 to 5 mm is uniformly formed in the front edge of the paper. In case of the image information the content of which provides an image output pattern like that shown in FIG. 7, it is necessary to minimize the blank space. Unless the image blank space is set to 2 mm or less, the image information will drop out. That is, for example, in the case where the image information to be generated exists up to the outer periphery of the paper as in case of the facsimile information, if the blank space is formed on the recording medium as mentioned above, important characters are not formed in the upper edge of the paper like an facsimile output paper or an image drops out so long as the upper edge of the facsimile output paper is set in accordance with the reference position of the original.
Further, in case of the image information which provides an image output pattern such as shown in FIG. 8 , when there is used a fixing method of an electrophotograph using a heat roller whereby the toner image is melted by heat and pressure of the fixing device and is bonded onto the paper, the paper, just after the toner image was fixed, performs a motion such that it is wrapped around a fixing roller (not shown) by a melt bonding force of the toner. The paper releases from the fixing roller as a result of its rigidity as compared to the wrapping force. However, in the case where a black image exists in the leading portion of the paper, particularly, the wrapping force further increases and, in the worst case, the paper is not separated from the fixing roller and a wrapping jam occurs in the fixing roller.
If the image blank amount is set to a value less than 5 mm in order to avoid such a situation, there is a contradictory problem such that when a painted image exists in the edge portion of the original, a fixing jam such that the recording medium is wrapped around the fixing roller as mentioned above or the like occurs.